1. FIELD OF THE INVENTION
This invention relates to improvements in systems and techniques for radio transmitting and receiving and to antenna systems for use is such radio systems, and, more particularly, to improvements in radio receiver antennas adapted for use in full duplex operation in proximity to a transmitter antenna.
2. BRIEF DESCRIPTION OF THE PRIOR ART
In many instances it is desirable to operate both a radio transmitter and receiver simultaneously, such operation being referred to herein as full duplex operation. Although such operation in the case of a receiver which happens to be located adjacent a transmitter antenna may be fortuitous, such operations are often times intended. One such instance in which such duplex operation may be desirable, for example, is in balloon-borne very low frequency (VLF) relay systems and the like. Typically, the transmitter and receiver in such systems operate near the same frequency, and in VLF systems, usually the transmitter and receiver utilize the same antenna. Because of the wave lengths involved in VLF operations, long wire or monopole vertical antennas are usually used, suspended, for instance, from a balloon or the like.
In the past, in order to enable the same antenna to be used for both transmitting and receiving, in some cases a notch filter was used to reduce the magnitude of the transmitter output at the receiver to enable the receiver to operate without its input being saturated. Notch filters typically employed, however, are difficult to design because they require a very high Q. (Notch filters typically employed often have insertion losses greater than 60dB at the transmitter center frequency whereas they may typically have 0 to 6dB loss at the adjacent receiver frequency.) This requirement is compounded if the transmitter is designed to operate over a band of frequencies, thus requiring the notch filter to be tuneable, or at least switchable, amongst various frequencies offset from the transmitter frequency.
Typically, a transmitter may be centrally located along the length of a long wire antenna, which may be several thousand feet in length, and which may be suspended from a balloon. The receiver derives its input from the long wire transmitter antenna by a current transformer, which may, for example, be an iron core ring surrounding the long wire and having multiple turns around the ring from which the output signal is derived for application to the aforementioned notch filter.
In light of the above, it is, therefore, an object of the invention to provide an antenna configuration which is essentially insensitive to magnetic fields generated in a long wire antenna by an associated transmitter, yet is sensitive to receive both TE and TM fields from distant transmissions.
It is another object of the invention to provide an antenna configuration of the type described for use in VLF applications.
It is yet another object of the invention to provide an antenna configuration of the type described for use in balloon-borne systems.
These and other objects, features and advantages will become apparent to those having ordinary skill in the art from the following detailed description when read in conjunction with the accompanying drawings and appended claims.
The invention in its broad aspect, presents an antenna system including a receiving antenna for use in proximity to a transmitting antenna having a circular radiating magnetic field. At least one receiving antenna has a null spatial sensitivity pattern along at least one axis, oriented with the transmitting antenna located in said null pattern. Thus, the at least one antenna is insensitive to signals from said transmitting antenna and is sensitive to signals from a remote location. In one embodiment, the receiving antenna includes at least one loop which is oriented with an imaginary line piercing a central point of the loop being perpendicular to a plane containing the loop. The line is disposed to intersect a monopole transmitter antenna, resulting in the monopole antenna being located in a null region of sensitivity of the loop antenna.